1. Field of the Invention
The present invention relates to a printing plate used in flexographic printing and, more particularly, it relates to a flexographic printing plate which is easily manufactured and capable of high quality printing.
2. Description of the Prior Art
A flexographic printing plate has been conventionally manufactured in the following manner. In the first method, the printing plate is formed by manually engraving a plate material of natural rubber or synthetic rubber. In the second method, it is manufactured by molding. That is, natural rubber, synthetic rubber or synthetic resin is poured into a mold made of a thermosetting resin. In the third method, the printing plate is formed by exposing a photosensitive resin with the desired regions masked. In the fourth method, which has become popular recently, the rubber material is engraved by an NC (numerical control) apparatus using laser light.
FIG. 1 is a plan view of a conventional flexographic printing plate and FIGS. 2A and 2B are cross-sectional views taken along the line II--II of FIG. 1. Referring to FIGS. 1, 2A and 2B, the conventional rubber plate comprises a convex portion 1 with a flat surface whose upper surface is to be the printing surface 5 and a support rubber portion 3 for supporting the convex portion 1. The support rubber portion 3 includes a textile layer for preventing the support rubber portion 3 from being stretched. The convex portion 1 may have a vertical edge at the top end surface thereof (FIG. 2A) or the convex portion may have a taper formed from the top end surface thereof (FIG. 2B).
The method of manually engraving the rubber plate will be described below.
When the rubber plate is directly engraved by manual operation, a pattern or character is transferred onto the surface of the rubber plate material with the scale thereof reduced in one direction. When the plate is mounted on the peripheral surface of a press cylinder, the printing plate extends in the peripheral direction. The above described reduction of the pattern, character and the like compensates for the resulting extension of the plate. Thereafter, a line is cut along the contour of the transferred pattern or the like by a pointed knife. The unnecessary portion on one side of the cut line is removed by an engraving knife. When the contour is cut by a pointed knife, there arises some problems if the surface of the plate material is vertically cut from above. Namely, if the width of the image area (i.e. relief) of the obtained printing plate is narrow and the height thereof is about 5 mm (the normal thickness of the plate material is 7 mm), then accurate printing cannot be obtained since the strength of the image area is not sufficient. More specifically, the image area cannot stand the pressure applied at the time of printing, and the image area deforms in the transverse direction. Therefore, the side surfaces 2 and 2' of the convex rubber 1 and 1' constituting the image area are conventionally formed with taper as shown in FIGS. 2A and 2B. The side surface of the image area is formed with a taper whether made by the molding method, the method utilizing the photosensitive resin, the method utilizing laser light.
Recently, a method for manufacturing a flexographic printing plate using an automatic drawing machine having an NC controlled cutting head has been practically used. The details of the automatic drawing machine are disclosed in the U.S. Patent Application Ser. No. 114,664 entitled "Apparatus for Automatically Providing Positioning Holes on Film Material" which is assigned to the owner of the present invention. In this case, two flat bodies of natural rubber, synthetic rubber or compound rubber (which is the mixture of the two) are used as the plate material. The two flat bodies are detachably bonded and the stacked flat bodies are cut by the above mentioned automatic drawing machine. The manufacturing method of the plate in this case is as follows.
First a plate material is placed on a cutting table of the automatic drawing machine. The cutting head is moved in the X and Y directions along the cutting table surface based on the NC data. Along with the above mentioned movement, the cutter blade mounted on the cutting head vertically cuts the upper layer flat body of the plate material from above to the interface between the upper and lower flat bodies. Thereafter, the unnecessary portion on one side of the cut line is removed manually or by simple instruments such as tweezers from the upper flat body.
The flexographic printing plate has been manufactured employing the above described methods. When the flexographic printing plate is manufactured by manually engraving the rubber plate material, the operator must be skilled and the operation efficiency is low. In addition, in this case, the cutting is carried out along the pattern, character and the like transferred onto the surface of the rubber plate material. The operator manually cuts the patterns or the like with visual observation. Therefore, the printing plate manufactured by this method is of questionable precision. This method cannot be used to manufacture printing plates with complicated image, areas. The method of manufacturing the printing plate by molding takes much time and it is a troublesome task to make a mold. The edge of the image area of the manufactured flexographic printing plate is not very acute. Therefore, when printing is carried out using the plate, high quality printing cannot be obtained. The printing plate manufactured by exposing the photosensitive resin with a predetermined mask is inferior to the rubber plate in durability for printing. In addition, the method is expensive and necessitates the use of large apparatus to manufacture the printing plate. In the method for manufacturing the flexographic printing plate utilizing laser light, the rubber plate material is directly processed based on the NC data. Therefore, it can save labor and reduce the time for manufacturing. However, in this case, the apparatus becomes large and complicated, increasing the cost of manufacturing the printing plate. FIG. 3 is a cross-sectional view of an image area of the flexographic printing plate manufactured by the automatic drawing machine. The flexographic printing plate manufactured by the automatic drawing machine comprises a convex rubber portion 1" constituting the image area (which is the remaining portion of the upper layer of the stacked plate material with the unnecessary portions removed), the support rubber portion 3' constituted by the lower layer of the plate material and a textile layer 4' embedded in the support rubber portion 3'. However, in the flexographic printing plate manufactured in this manner, it is difficult to provide a tapered side surface to the convex rubber portion 1". Therefore, the strength of the printing plate becomes insufficient as the width of the convex rubber portion 1" constituting the image area becomes thinner. The printing plate cannot withstand the pressure at printing. Consequently, the convex rubber portion 1" is deformed in the transverse direction, preventing proper contact between the printing surface and the surface to be printed.